The present invention relates to magnetostrictive composites which are effective for use as magnetostrictive sensors and actuators. More particularly, the magnetostrictive composites include metal oxide of the ferrite type and a metallic binder. The metal oxide of the ferrite type and metallic binder provides magnetostrictive and mechanical properties that make the composites effective for use in a wide variety of applications.
The magnetic properties of many ferromagnetic materials undergo changes with stress. For example, the magnetic permeability of nickel-iron alloys and iron-cobalt alloys increases and that of nickel decreases with tensile stress. Conversely, if these metals are subject to magnetic fields, their dimensions can change. These magnetostrictive effects, including the Joule effect (change in length when a ferromagnetic rod is placed in a longitudinal field) and the Villari effect (change in magnetization when a magnetized ferromagnetic rod is subjected to longitudinal stress), can be used for converting electrical power to mechanical power and vice versa. Examples of the use of ferromagnetic materials include sensors (U.S. Pat. Nos. 4,414,510 and 5,442,966), transducers (U.S. Pat. No. 3,753,058), and vibrators (U.S. Pat. No. 4,151,432). These types of sensors have low sensitivity (""966 patent) or measure applied magnetic field (""510 patent) rather than stress.
The magnetomechanical effect can be explained in terms of domain processes in magnetostrictive materials. It is known that torsional stresses on a cylinder can be considered to be a biaxial stresses, in which the two stress axes are perpendicular to each other and at 45xc2x0 to the torsion axis. The stresses along these axes are equal in magnitude but opposite in sign (Sablik et al, IEEE Tans. Magn. 35, 498 (1999)). Due to negative magnetostriction, the net magnetization of Co-ferrite composites tends to rotate towards to the compressive stress direction (Garshelis, IEEE Trans. Magn. 28, 2202 (1992)).
The stresses have two effects on domain wall motion. From a thermodynamic viewpoint, the effect of the stresses can be considered as an effective magnetic field (Jiles, J. Phys. D: Appl. Phys. 28, 1537-1546 (1995)), which induces pressure on 90xc2x0 domain walls in magnetostrictive materials and leads to domain wall motion. Domain wall motion can be either reversible or irreversible, depending on the strength of the domain wall pinning. If a domain wall is strongly pinned at some points but relatively free to move in-between, application of stress can cause it to bow, much like an elastic membrane. When the stress is removed, it can return to its original position, thus giving reversible behavior. It is the reversible part of the magnetoelastic coupling that determines the sensitivity of the magnetomechanical sensor material. However, if this effective field is strong enough to release a domain wall from a pinning site and move it ahead to another location, the domain wall will not return when the stress is removed, thus causing irreversible changes in magnetization, and hysteresis in the magnetomechanical response. Magnetic anisotropy and local variations of magnetic properties contribute to this hysteretic behavior also. Anisotropy and imperfections in magnetic materials inhibit the changes in magnetization as it attempts to approach the thermodynamic equilibrium state. This causes hysteresis in the present magnetic torque sensor measurement.
The magnetomechanical effect can be discussed in terms of Le Chatelier""s principle assuming the process to be reversible. For small reversible changes of magnetization, a thermodynamic relation exists, namely             (                        ⅆ          λ                          ⅆ          H                    )        σ    =            (                        ⅆ          B                          ⅆ          σ                    )        H  
where xcex is the magnetostriction (strain), H is the magnetic field, B is the magnetic induction and "sgr" is the stress. Thus, a reversible relationship between magnetostriction and magneto-mechanical effect in response to torsional stress also exists, which has some similarities to the thermodynamic relation. The sensitivity of magnetization to small applied torsional stress (less than 10 Nxc2x7m) depends on the piezomagnetic coefficient d33, (dxcex/dH)"sgr", rather than the saturation magnetostriction xcexs. In view of this, a high d coefficient, rather than a high saturation magnetostriction is the critical factor in selecting materials for magnetic torque sensors.
There has been considerable interest in using the magnetomechanical effect in sensors where stress is converted into a change in the magnetization of the magnetostrictive material. Any change in magnetization can be sensed without making contact with the sample. Such a sensor would be ideal for measuring torque in a rotating shaft such as in a drive train or power steering application. There is significant incentive to develop such a torque sensor for power steering applications as the parasitic losses associated with the hydraulic pump in existing power steering systems are relatively large. For example, an electronic based system will result in a five percent decrease in fuel consumption.
Terfenol (commercially available as Terfenol D from Edge Technologies, Ames, Iowa), which is an alloy of terbium, iron and dysprosium, is an excellent magnetostrictive material, however, it has a number of shortcomings. For some applications, Terfernol is not economically viable as a result of the high costs of terbium and dysprosium. Further, in order to obtain optimal results a single crystal is required. As a rare earth transition metal intermetallic, containing high levels of rare earths, Terfenol is extremely brittle and the high content of rare earth metal makes the material extremely susceptible to corrosion.
Other compounds such as nickel and maraging steel have also been considered for use as magnetostrictive sensor material. Nickel has good corrosion resistance and moderate costs, but has only moderate magnetostriction. Maraging steel is lower in cost and has a lower magnetostriction than nickel, but requires carefully controlled heat treatment to produce optimum magnetostriction.
Attempts have been made to use metal oxides of the ferrite type as magnetostrictive vibrators (U.S. Pat. No. 4,151,432). For example, the ""432 patent describes a macroscopically homogenous sintered ferrite structure that is Fe3O4 to Fe2O4.1 either alone or in combination with Fe2O3. These types of composites have been found to be unsuitable for use in brazing, a technique that could be a preferred method used to attach the sensor material onto a functional component such as a steering or drive shaft.
The present invention is directed to magnetostrictive composites which are effective for use as magnetostrictive sensors and actuators. The magnetostrictive composites of the invention have excellent corrosion resistance and mechanical properties that make them useful in a number of applications, including their use as sensors in the automotive industry. Further, the magnetostrictive composites of the invention provide economic advantages over other materials used as sensors.
In an important aspect of the invention, the magnetostrictive composites include metal oxide of the ferrite type having a density of at least about 80% of its maximum theoretical density. The metal oxide of the ferrite type of the invention has the general formula MexFeyO4, where Me is Mn, Co, Ti, Zn and mixtures thereof, wherein x is about 0.04 to about 1.3 and y is about 1.7 to about 3.
In another important aspect of the invention, the mechanical properties, the braze-ability, and the sensitivity of the magnetostrictive composites may be improved by blending the metal oxide of the ferrite type with a metallic binder and heating and/or pressing in order to produce a ceramic metallic composite. Metallic binder includes transition metal, silver, or a mixture thereof, and may further include alloys having the general formula Ag1xe2x88x92xNix, Ag1xe2x88x92Coy, where x is about 0 to about 1.0, Ag1xe2x88x92xxe2x88x92yNixCoy, where x+y is about 0 to about 1.0, or binders of the general formula plus other metallic additions which total less than about 50 weight percent. Transition metals useful in the present invention may include Co, Cr, Mn, Fe, Ni, Ti, Cu, Zn and any mixtures thereof.
In this aspect of the invention, the ceramic metallic composite includes a metal oxide of the ferrite type and a metallic binder in amounts which provide a ceramic metallic composite having a density of at least about 70% of its maximum theoretical density. The ceramic metallic composite is effective for providing an amplitude of magnetostriction of at least about 10 to about 400 ppm and is effective for providing a material that has a fracture strength of at least about 10 KSI (kilopounds per square inch).
In a very important aspect of the invention, the volume ratio of metal oxide of the ferrite type to metallic binder can be varied from about 4:1 to about 99:1. The metallic binder does not result in removal of an amount of oxygen from the oxide ferrite ceramic that would degrade or reduce magnetostrictive properties, but acts to wet the ceramic in order to hold the ceramic particles together and to improve the mechanical properties of the material in its solid form. The alloys of the general formula useful in the present invention have a melting point between about 900xc2x0 C. and about 1400xc2x0 C. and may include silver/nickel, silver/cobalt, silver/copper/nickel, and silver/copper/zinc/nickel.
In another aspect of the invention, the ceramic metallic composite may further include a hard magnetic powder such as a hard ferrite. Examples of hard ferrites include compounds of the following composition,
MO+6Fe2O3
where M is barium, strontium or a combination of the two. In this aspect of the invention, the ceramic metallic composite may include from about 1 to about 50 weight percent hard magnetic powder, based on the weight of the ceramic metallic composite. This serves to provide a magnetic bias field to the magnetostrictive component of the composite.
The present invention further provides a method for producing magnetostrictive composites. In an important aspect of the invention, metal oxides are blended and reacted to form a metal oxide of the ferrite type having a particle size of about 0.01 to about 50 microns. Metal oxides useful in the present invention may include cobalt oxide, iron oxide, manganese oxide, titanium oxide, zinc oxide, and mixtures thereof. The metal oxide of the ferrite type may be blended with a metallic binder that includes transition metals, silver, or a mixture thereof or an alloy having the general formula Ag1xe2x88x92xNix, Ag1xe2x88x92xCox, where x is about 0 to about 1.0, Ag1xe2x88x92xxe2x88x92yNixCoy, where x+y is about 0 to about 1.0, or binders of the general formula plus other metallic additions which total less than about 50 weight percent.
The handling of the resulting part formed from the metal oxide/metallic binder blend may be improved by adding a resin to improve green strength. Resin is burned out at a lower temperature and the green body is then sintered at a temperature of from about 600xc2x0 C. to about 1200xc2x0 C. in air for about 1 to about 30 minutes to provide a finished ceramic metallic composite. Resins useful in the present invention include Plenco resin, other commercial binders manufactured for this purpose, and mixtures thereof.